With development of communications technologies, providing a service to multiple terminals (User Equipment, UE) simultaneously by using an orthogonal multiple access (OMA) technology becomes an indispensable function of a cellular communications system. A basic idea of the OMA technology is: partitioning a resource into several mutually orthogonal resource blocks and allocating the resource blocks to different UEs. From a perspective of an information theory, an OMA mechanism is strictly suboptimal in most cases. For detailed analysis, refer to the preface of DESCRIPTION OF EMBODIMENTS, and no details are repeated herein.
It can be known, through analysis, that a transmit rate that can be reached in a superposition coding-successive decoding (SC-SD) mechanism is strictly higher than that in the OMA mechanism. Inspired by the SC-SD mechanism, DoCoMo proposes a multiple access technology named non-orthogonal multiple access (NOMA), where the technology is completely the same as the SC-SD mechanism during an uplink, and therefore, no details are repeated. However, during a downlink, if an operation is intended to be performed successfully, a condition |h1|≤|h2| needs to be satisfied. If |h1|>|h2|, a message m1 cannot be detected correctly at a terminal 2. Wrong decoding results in error propagation inevitably. In this case, a message m2 also cannot be detected correctly.
Therefore, it can be seen that in the NOMA technology, a UE detection sequence is specified, that is, a UE under a worse channel condition needs to be detected first, and then a UE under a better channel condition is detected. In this case, a transmit rate of the UE that is under the worse channel condition is reduced. A signal from the UE that is under the better channel condition needs to be regarded as noise during detection, which is equivalent to reducing a signal-to-interference-plus-noise ratio (SINR). Therefore, to ensure that a message can be detected correctly, a code rate needs to be reduced. That is, although a sum of rates is increased, what is increased is mainly a rate of the UE that is under the better channel condition. This is unfair to the UE that is under the worse channel condition.